1. Field of the Invention
The invention relates to a method and apparatus for controlling the addition of auxiliary fuel to a two-stage combustion furnace system which is operated in the pyrolysis (starved-air) mode in the first stage and in the excess air mode in the second stage.
2. Information Disclosure Statement
The incineration of combustible materials, especially waste materials such as sewage sludge two-stage "starved air" furnace systems is well known. In such furnace systems, combustible materials are incinerated under "starved air" conditions in a first stage to produce partially oxidized, combustible gases and vapors which are subsequently carried into a secondary stage where they are combusted with excess air.
An example of such two-stage incineration for incinerating sludge is a multiple hearth furnace equipped with an afterburner. In the multiple hearth furnace, the waste is pyrolized in an oxygen deficient atmosphere (i.e. under starved air conditions), which is desirably regulated to only partially complete the oxidation of the organic substances pyrolyzed from the waste. In the afterburner, air is introduced to complete the oxidation of the substances pyrolyzed from the waste in the furnace. The air supplied to the afterburner is controlled so that at temperatures above a predetermined temperature, the quantity of air introduced is increased with increasing temperatures and is decreased with decreasing temperature. In other words, the pyrolyzing furnace is caused to operate with a deficiency of air over its operating range, while the afterburner is caused to operate with excess air, i.e. above the stoichiometric value, and the amount of excess air supplied may be used not only to complete combustion but to control the operating temperatures by quenching. Examples of such two-stage systems may be found in U.S. Pat. Nos. Re 31,046, 4,182,246, 4,046,085 and 4,050,389.
As just described, when the net heating value of the waste is insufficient to maintain the desired first stage temperature, the control system will tend to increase the first stage air rate into an excess air condition, which is undesireable. Furthermore, as long as the temperature is below the set-point, the air rate will continually increase. Such increase under excess air conditions will cool rather than heat the first stage.
In reality, of course, auxiliary fuel burners are used to supplement the waste-generated heat. In order to prevent the first stage from becoming super-stoichiometric with regard to air, the auxiliary burners continuously operate at a rate which exceeds the maximum expected deficit in fuel requirement. Such operation is extremely wasteful of fuel, particularly when the feed material is usually close to or in excess of the autogenous heating value.
The problem just mentioned is addressed in co-pending U.S. patent application Ser. No. 333,102 of Lewis, filed Dec. 21, 1981. The air rate to the first stage is not allowed to exceed a pre-determined percentage of the stoichiometric rate. In other words, the first stage or primary air rate is "clamped" at a particular percentage of the stoichiometric value. In practice it would rarely be economically advantageous to operate at or close to the clamping value of percent stoichiometric air.
Still remaining, of course, are the questions of when and where the auxiliary burners should be fired. In addition, since both added air and added auxiliary fuel will increase the first stage temperature (provided the stage is in a sub-stoichiometric condition), these heat-generating steps must be continually balanced, preferably at the most economic ratio.
The degree of oxidation in the first stage will affect the quality of auxiliary fuel (if any) required to maintain the proper second stage temperature. From thermodynamic considerations it is preferred that auxiliary fuel be added to the first rather than the second stage of such two-stage furnaces. If the first stage requires auxiliary heat, the second stage generally will also. Heat supplied to the first stage is carried into the second stage.
In waste treatment applications, the terms "starved-air" and "pyrolysis" are generally applied to two-stage furnace systems, even though the first stage only is operated with less than stoichiometric air rate, and the system as a whole is fed excess air.
Furthermore, even though the terms "starved-air" and "sub-stoichiometric air" are technically more correct than "pyrolysis" in regards to the operation of the first stage, the terms will be used interchangeably in this application.
One method of illustrating the background thermodynamic principles which govern continuous combustion processes is through the use of graphs as in FIGS. 1-4, in which temperature is plotted as a function of (a) air rate or (b) percent stoichiometric air rate. The latter is the absolute air rate divided by the stoichiometric air rate required for complete combustion multiplied by 100. Furnaces for destroying waste materials are typically operated at 150+ Percent Stoichiometric Air in order to ensure complete combustion under varying feed rates, heating values and feed moisture content.
A typical graph for combustion of dry wood is shown as the upper line in FIG. 1. All of the points to the right of 100% stoichiometric air are computed using a conventional heat and material balance. When the primary combustion chamber is operated in the starved air (less than 100% stoichiometric air) mode, a combustible gas, containing carbon monoxide, hydrogen, methane, higher order hydrocarbons, along with some tars and oils, will be produced. These combustible gases are generated by the process of destructive distillation. The reactions are both endothermic and exothermic, and the exact shape of the curve in the starved air region is difficult to determine. However, for design purposes, a straight line between the known points 0% and 100% stoichiometric air is adequate.
A more typical waste material would contain moisture, and a curve for a 70% moisture wood is also shown in FIG. 1. Before a fraction of the combustible material can be reacted, all of the moisture must be evaporated (a wet ash should never leave the furnace) and this evaporation of moisture requires a significant amount of heat. In starved air operation, the quantity of air is directly proportional to the quantity of combustible material reacted. For the 70% moisture wood, slightly over 50% of the combustible material (50% stoichiometric air) must be reacted to have all of the moisture evaporated at 212.degree. F. Typical first stage and afterburner operating points are indicated on this lower curve. The first stage is shown operating at 75% stoichiometric air with an exit temperature of 1,000.degree. F., and the afterburner is being operated at a temperature of approximately 1,500.degree. F. In the language of the industry, it would be stated that the afterburner is being operated at 150 percent stoichiometric (that is, 50 percent excess) air. Of course, it is more accurate to say that the furnace system, as a whole, is operating at 150 percent stoichiometric air.
In further illustrating the background to the present invention, FIG. 2 shows a similar curve for a sewage sludge with the following specific characteristics and furnace operation:
______________________________________ Wet Feed Rate 23600 lb/hr Moisture Content 73% Combustible Content (Dry Basis) 65.4% High Heating Value of Combustibles 12000 BTU/lb Combustible Elemental Analysis C 57.33% H 8.13% S 1.24% O 28.45% N 4.85% Total 100.00% ______________________________________
The calculations include heat losses by radiation and convection, heat loss associated with the combustible material which will remain in the ash, and the heat loss from the sensible heat in the ash.
It should be recognized that the curve for actual waste streams such as partially dewatered sewage sludge varies from instant to instant. Higher heating values and/or less moisture will affect the curve on either or both sides of the 100 percent stoichiometric value.
FIG. 3 shows combustion curves for the same sludge as in FIG. 2. The "NO FUEL" line is identical to the curve of FIG. 2. and represents the sludge alone, without any auxiliary fuel. The maximum temperature achievable with this sludge alone is about 1460.degree. F. If the first stage is operated at 1400.degree. F., an actual air rate of 32,000 pounds per hour is 97 percent of the stoichiometric rate of 33,000 pounds air per hour. This "percent stoichiometric air" is considerably higher than the exemplary desired value of 90 percent. The desired first stage temperature is 1400.degree. F. and desired percent stoichiometric air of 90 percent can only be achieved by introducing and combusting an auxiliary fuel in the first stage. In this example auxiliary fuel is also required in the afterburner. The total auxiliary fuel used to achieve 1400.degree. F. furnace offgas temperature is 5.58 million BTU/hr. The fuel addition to the afterburner needed to maintain a 1400.degree. F. offgas temperature is 0.36 million BTU/hr for a total of 5.94 million BTU/hr of auxiliary fuel. The "combustion path" is indicated on the figure. It can be seen that the percent stoichiometric air is now 34,000 pounds per hour divided by 37,800 pounds per hour, times 100, or 90 percent. The total air rate to both stages is shown to be 53,000 pounds per hour (140 percent of stoichiometric) and the afterburner temperature is controlled at 1400.degree. F.
FIG. 4 is a replot of FIG. 3 where the Percent Stoichiometric Air is used as the abscissa rather than the air rate. The set point of 90% stoichiometric air to the furnace is used to obtain a 1400.degree. F. furnace temperature as indicated.
The effects of fuel, air, and combustible waste characteristics upon the operation of any furnace can be clearly visualized from such an analysis.
It is an object of this invention to provide a two-stage "starved-air" furnace system capable of efficiently combusting waste materials of varying heating value and moisture.
A further object of this invention is to provide a furnace system in which the primary stage is maintained in the "substoichiometric air" mode, despite large variations in feed rate, moisture contents and heating value.
A further object is to provide a furnace in which auxiliary fuel is preferentially supplied to the first stage rather than the second stage, in order to achieve the most efficient use of the auxiliary fuel.
Yet another object is to maintain temperatures in both stages at relatively uniform levels.
A further object is to provide a furnace in which the air rate to the primary stage is maintained at a uniform fraction of the stoichiometric requirement, despite rapid changes in the absolute value of the stoichiometric requirement.
Another object is to provide a furnace in which the control is based on criteria which are easily measured on a continuous basis.
It is an additional object of the present invention to provide an improved method of controlling the incineration of combustible materials in the starved-air mode which enables operation of the primary stage close to the stoichiometric point, and maintaining an identifiable safety margin to prevent instability problems.